The present invention relates generally to computed tomography (CT) scanners, and more specifically to a method and apparatus to verify that a CT scanner meets its performance specifications.
For convenience of exposition, the following notations are used in the following specification:
AD Automatic Detection
CT Computed Tomography
FOV Field-of-view
IQP Image Quality Phantom
NSR Nutating Slice Reconstruction
ROI Region of Interest
SSP Slice Sensitivity Function (Profile)
MTF Modulation Transfer Function
z-axis Coordinate axis that coincides with axis of gantry rotation. The positive direction is the same as the direction of the gantry angular velocity vector.
x-axis Horizontal coordinate axis. The positive direction is to the right side of the gantry as viewed from the front face.
y-axis Vertical coordinate axis. The positive direction is upwards of the gantry is viewed from the front face. The x, y and z axes form a right handed system.
Axial slice Slice perpendicular to the z-axis
Sagittal slice Slice perpendicular to the x-axis
Coronal slice Slice perpendicular to the y-axis
A CT scanner is a device used for manual or automatic discrimination of compositions, conditions or objects. In addition to traditional applications in medical imaging, non-medical applications for CT scanners are evolving. For example, baggage scanners using CT techniques have been proposed to search for contraband items such as explosives and narcotics in luggage at airports. Scanners for industrial testing have also been proposed.
One type of system using CT technology is a CT scanner of the third generation type, which typically includes an X-ray source and an X-ray detector system secured to diametrically opposite sides of an annular-shaped platform or disk. The disk is rotatably mounted within a gantry support so that in operation the disk continuously rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system can include a linear array of detectors disposed as a single row in the shape of a circular arc having a center of curvature at the focal spot of the X-ray source, i.e., the point within the X-ray source from which the X-rays emanate. Alternatively, the detector system can include a xe2x80x9ctwo-dimensionalxe2x80x9d array of detectors disposed as multiple rows forming a cylindrical surface whose axis passes through the source. The X-ray source generates a fan-shaped beam (when used with a linear array of detectors) or cone-shaped beam (when used with a two-dimensional array of detectors) of X-rays that emanates from the focal spot, passes through an imaging field, and is received by the detectors. The CT scanner includes a predefined coordinate system, defined by mutually orthogonal X-, Y- and Z-axes, wherein the axes intersect and are all normal to one another at the center of rotation of the disk as the disk rotates about the rotation axis. This center of rotation is commonly referred to as the isocenter. The Z-axis is defined by the rotation axis of the scanner, and the X-and Y-axes intersect the Z-axis at the isocenter and are defined by and lie within the planar imaging field. The X, Y and Z-axes form a right handed system. The fan beam is thus defined as the volume of space existing between a point source, i.e., the focal spot, and the receiving surfaces of the detectors of the linear detector array exposed to the X-ray beam. In the case of the fan beam, because the dimension of the receiving surfaces of the linear array of detectors is relatively small in the Z-axis direction the fan beam is relatively thin in that direction. In a similar manner, the cone beam is defined as the volume of space existing between a point source, i.e., the focal spot, and the receiving surfaces of the detectors of the detector array exposed to the X-ray beam. Scanners have been developed for generating three dimensional images from the data acquired from a scan.
Each detector generates an output signal representative of the intensity of the X-rays incident on that detector during a sampling period. Since the X-rays are partially attenuated by all the mass in their path, the output signal generated by each detector is representative of the density of all the mass disposed in the imaging field between the X-ray source and that detector.
As the disk rotates, the detectors of the detector array are periodically sampled, and for each measuring interval each of the detectors in the detector array generates an output signal representative of the density of a portion of the object being scanned during that interval. The collection of all of the output signals generated by all the detectors of the detector array for any measuring interval is referred to as a xe2x80x9cprojectionxe2x80x9d, and the angular orientation of the disk (and the corresponding angular orientations of the X-ray source and the detector array) during generation of a projection is referred to as the xe2x80x9cprojection anglexe2x80x9d. At each projection angle, the path of the X-rays from the focal spot to each detector, called a xe2x80x9crayxe2x80x9d, increases in cross section from a point source to the receiving surface area of the detector, and thus is thought to magnify the density measurement because the receiving surface area of the detector area is larger than any cross sectional area of the object through which the ray passes.
As the disk rotates around the object being scanned, the scanner generates a plurality of projections at a corresponding plurality of projection angles. Using well known algorithms, a CT image of the object may be generated from all the projection data collected at each of the projection angles. Where the object is stationary during the scan (so called xe2x80x9cconstant axisxe2x80x9d scanning), the CT image is representative of the density of a xe2x80x9ctwo dimensional slicexe2x80x9d of the object through which the X-ray beam has passed during the rotation of the disk through the various projection angles. When the object and rotating disk are moved relative to one another along the Z-axis (so called xe2x80x9chelicalxe2x80x9d or xe2x80x9cvolumetricxe2x80x9d scanning), a collection of data is acquired though a volumetric xe2x80x9cslicexe2x80x9d of the object through which the X-ray beam has passed during the rotation of the disk through the various projection angles. Multiple slices can be obtained in a xe2x80x9cstep-and-shootxe2x80x9d process, a mode of operation where successive xe2x80x9caxialxe2x80x9d slices are obtained from constant axis scans respectively at incremental positions of the gantry relative to the scanned object. Thus, step-and-shoot scanning is a mode of operation of the scanner in which projections for one axial slice or set of axial slices, are acquired without translating the object or patient and gantry during each axial scan. To get projections of another slice or set of slices, the patient and gantry are translated relative to one another before the next acquisition begins. There is no simultaneous imaging and translation as in helical scanning. Data from successive slices obtained from a step-and-shoot scan can be utilized to provide a volumetric image. The resolution of the CT image is determined in part by the width of the receiving surface area of each detector in the plane of the fan beam, the width of the detector being defined herein as the dimension measured in the same direction as the width of the fan beam, while the length of the detector is defined herein as the dimension measured in a direction normal to the fan beam parallel to the rotation or Z-axis of the scanner.
Image processing techniques are known in the art for generating an image of the target object, slice by slice. Each slice is viewed as being composed of a plurality of individual volume elements. Information regarding the different amounts of x-ray attenuation by the different volume elements within each slice is used to determine the density and position of the internal structures that make up the slice. Each volume element is characterized by a numerical value, referred to as the CT number, which represents its attenuation characteristics. CT numbers are conventionally scaled relative to the x-ray attenuation coefficient of pure water, which is assigned a CT number equal to 0 under the Hounsfield scale that ranges from low density (about xe2x88x921000) to high density (about +3095). The CT number of a material thus represents the attenuation coefficient of the material relative to the attenuation coefficient (0) of pure water. Soft tissues commonly have CT numbers in the range from about xe2x88x921000 to about 500. The CT number for bone is about 800, whereas metals often have CT numbers in excess of 2000.
In some applications, it has been beneficial for the CT system equipment to automatically analyze the acquired density data and determine if the data indicate the presence of any contraband items, e.g., explosives in the case of non-medical applications, or any abnormality in the case of medical application. This automatic detection process should have a relatively high detection rate. At the same time, the false alarm rate of the system should be relatively low to substantially reduce or eliminate false alarms on innocuous items or conditions.
Systems used for discrimination of compositions, objects or conditions must provide the same (i.e. consistent and uniform) discrimination results for the same scanned compositions, objects and conditions, so that one set of algorithms and parameters may be used by all of the scanners. This requirement applies to any composition, object or condition that may be scanned. Therefore, the images provided by the scanners should be similar enough that the same discrimination results are obtained.
In order to ensure that a scanner meets specification in terms of its discrimination capabilities and other performance criteria, it is beneficial to test its image quality. It is additionally beneficial to perform automatic image quality testing on the scanner. Finally, it is beneficial to test the image quality performance in a mode in which the scanner is normally operated. A normal operating mode, also referred to as normal mode, is defined herein as the mode of operation of the scanner for its application. For example, in a medical scanner, for some applications, a scanner may be operated with some protocol comprising some combination of helical scanning pitch, gantry speed, dose rate and reconstruction method. If the image quality can not be assessed in the normal mode of operation, or if there are several normal modes that produce images with different image quality, and only one is tested, then the performance may not be accurately assessed. A non-normal mode may have different scanning pitch, gantry speed, use a different reconstruction algorithm and/or different reconstruction parameters.
Specific phantoms are known to be used to evaluate the performance of CT scanners in terms of image quality. All the phantoms are designed for step-and-shoot scanning. There is no simultaneous imaging and translation as in helical scanning.
A common type of phantom, known as the xe2x80x9cwedding cakexe2x80x9d phantom, comprises multiple stacked disks of different diameters. Each disk contains inserts for specific measures of image quality. The various inserts, such as comb phantoms, wires, and low contrast objects, are in separate disks so that they are scanned separately, to prevent artifacts caused by one insert from corrupting the measurements of another insert.
The image quality assessment method by Engel et al., entitled xe2x80x9cComputerized Tomography Calibrator,xe2x80x9d U.S. Pat. No. 5,056,130, and a method by Aufrichtig et al., entitled xe2x80x9cApparatus and Methods for Automatic Monitoring and Assessment of Image Quality in X-ray systems,xe2x80x9d U.S. Pat. No. 5,841,835, represent , prior art in the field of image quality assessment. These methods require precise positioning of the phantom, step-and-shoot scanning, and special, non-normal modes of operation. These are some of the problems addressed by the present invention.
The present invention is directed to an apparatus for and method of processing computed tomography data for a region to assess the performance of the scanner in any of its normal modes of operation.
In one embodiment, the scanner is an explosives detection scanner (xe2x80x9cEDSxe2x80x9d), which has only one mode of normal operation, with a conveyor belt operating at a fixed conveyor speed (for transporting objects through the scanner), a disk rotating at a substantially fixed gantry speed, and the software including a fixed reconstruction algorithm and parameters. The image quality phantom (IQP) is manually placed on the conveyor belt of the EDS scanner for scanning, such that precise positioning of the phantom is neither possible nor necessary.
The image quality phantom is of a predetermined shape, configuration and composition, with a plurality of predefine inserts. The inserts are specially designed so that certain properties can be measured from CT images.
In one embodiment the phantom is scanned so as to reconstruct a 3-dimensional image from CT data of the phantom, and using an auto-detection (AD) computer, objects are identified and matched with the inserts, properties of the identified objects are measured, and the identified objects are compared with the known properties of the known inserts, so that the ability of the CT scanner to faithfully capture the properties can be assessed.
In one embodiment of the invention, the assessment verifies the following:
1. The stability of the high voltage power supply that powers the X-ray tube (hereinafter referred to as xe2x80x9cX-ray tube voltagexe2x80x9d).
2. The X-ray flux.
3. The placement of the conveyor belt relative to the scanner.
4. The speed of the gantry.
5. The speed of the belt.
6. The communication with the AD computer.
7. The AD computer discrimination performance including its parameters.
In one embodiment the IQP comprises a suitcase preferably made of plastic with inserts that are specially designed for the tests listed above. The inserts preferably comprise objects of known size, shape and density, for example, cylinders of Nylon, Teflon and PVC, comb phantoms, a diagonal rod and a sheet explosive simulant. The object sizes are selected for the fixed pixel size of the CT image, which is determined by the normal mode of scanner operation. The objects for measuring CT number stability and noise are preferably cylindrical to make them suitable for the lack of precise positioning.
In one embodiment, the X-ray tube voltage stability is verified by measuring the CT numbers corresponding to the various known densities of the objects provided in the phantom, e.g., Nylon, PVC and Teflon are suitable materials. The placement of the conveyor belt within the field of view is preferably measured by finding the position and orientation of the IQP within the reconstructed images. The photon flux is preferably verified by measuring noise in the difference between two images of one of the types of objects, e.g., Nylon. The communication is preferably verified by ensuring that no slices are missing using a specific type of object whose relative position with a slice image changes with each successive slice. For example, a diagonal rod whose position in each successive slice can be predicted with the knowledge of the phantom and its position in each slice image. The AD computer discrimination is preferably verified by running AD software on the images, and using simulants that exercise the pathways of the AD software. In one embodiment, the entire assessment is done automatically on a computer, although it can be done manually. The software identifies the location of the IQP in the image, and determines the validity of the scan upon which to base the conclusions about the scanner.
In one embodiment, the system identifies multiple objects within the 3-D image, by combining voxels that match in CT number and locations. Regions of interest within these objects are identified by the software which are used to measure the properties of mean and standard deviation. The system preferably identifies the axial and sagittal slices within which to measure Slice Sensitivity Function (Profile) (hereinafter xe2x80x9cSSFxe2x80x9d) and Modulation Transfer Function (hereinafter xe2x80x9cMTFxe2x80x9d), and measures the SSP and MTF by measuring the modulation that is obtained in the slices.